1. Field of the Invention
This application relates generally to optical diagnostic methods and apparatus and, more specifically, to an optic neuropathy detection method and apparatus utilizing optical coherence tomography.
2. Description of Related Art
Conventional diagnostics for eye disorders typically include a detailed ophthalmic examination of the retina. For initial examination, an eye doctor will view the retina through an ophthalmoscope. For a permanent record, the retina is typically photographed with a fundus camera. A fundus photograph directly records various anatomical features of the retina, such as the optic disc, fovea, blood vessels, and lesions. The imaging capabilities of fundus photography may be enhanced by supplementary techniques. A high-contrast image of retinal blood vessels, for example, can be photographed after the injection of a fluorescent dye into the bloodstream. The resulting image is referred to as a fluorescein angiogram.
More sophisticated techniques have been developed for diagnostics of the eye. One such technique is three-dimensional optical coherence tomography (3D OCT). In this technique, a light beam is directed onto the retina. Part of the beam is back-reflected, and interferometric analysis of the back-reflected light yields information on the structure of the retina. By varying optical parameters of the light probe, features at different depths below the surface of the retina may be analyzed. With this process, an image of a cross-section of the retina may be generated by scanning the optical probe along a line on the retina. By rastering the optical probe across the surface of the retina, a series of cross-sectional images may be produced. The series of cross-sectional images may be used to characterize the 3D structure of the retina, and parameters, such as local retinal thickness, etc., may be measured by 3D OCT.
Analysis of the thickness of the retina can be used to diagnose certain diseases of the eye, such as glaucoma. One indication of the health of the eye can be provided by comparing the retinal thickness of the patient's eye with reference data acquired from a population of healthy eyes. Progression of eye disease can also be monitored by measuring changes in retinal thickness over a period of time.
For example, a conventional approach is to utilize a circumpapillary scan to detect glaucoma and monitor a progression of glaucoma in a patient. A circle scan is performed around the optic disc at a fixed diameter. From the scan data, a thickness of the retinal nerve fiber layer (NFL) around the circle is measured.
Attempts have been made to derive other measurements which can be applicable to detecting optic neuropathy, such as glaucoma. Specifically, focus has been on measurements that can indicate glaucoma prior to measurable changes in retinal layer thicknesses. In one proposal, optical properties of the retinal NFL are calculated. In particular, an attenuation coefficient, which characterizes how rapidly intensity of light is attenuated as the light propagates through a medium (e.g., a scattering or absorbing medium), is one such optical property. It is suspected that changes in the attenuation coefficients of the retinal nerve fiber layer tissue can be detected prior to observable thinning typical with glaucoma.
However, attenuation coefficients are highly dependent on location, including in the axial direction, such that the attenuation coefficient can vary even within a particular tissue layer. Accordingly, an average coefficient value, e.g. calculated over a depth or thickness of the tissue layer but corresponding to a fixed and predefined depth such as the axial dimension length associated with a single pixel, is often utilized as a representative value for the tissue layer for a given X-Y location (A-line). Thus, this locationality of attenuation coefficients results in an indirect or proxy relationship to underlying physical reality associated with optic neuropathies. For instance, the attenuation calculation described produces results which are decorrelated from thickness. While attenuation coefficient measurements could correlate with a diagnosis of glaucoma, a thickness-based technique may be equally, and possibly more, effective.